Polyisocyanates are well-known precursors used in the manufacture of polyurethanes. In the past, commercial processes for the production of isocyanates have relied heavily on a phosgene-based production process. Unfortunately, phosgene is a toxic material that must be handled with caution. Accordingly, the industry has sought non-phosgene routes for the production of isocyanates.
Various methods for producing isocyanates by subjecting amides to modified Hofmann reactions are known in the art. The Hofmann reaction is "modified" in the sense that the reaction product is an organic isocyanate rather than an amine. As an illustrative example of such a reaction, U.S. Pat. No. 4,282,167 discloses using a 2-phase aqueous/organic system. Unfortunately this system is not as adaptable as might be desired, particulary since two-phase systems typically require the presence of water, and many isocyanates are water sensitive and must be produced in the absence of water. In addition, the disclosures of the '167 patent indicate that good product yields were only obtained for isocyanates with the isocyanate groups bonded to a secondary or a tertiary carbon. For others with the isocyanate groups bonded to a primary carbon, the yields were low, generally between 0% and 20%.
The production of isocyanates using a modified Hofmann reaction under anhydrous conditions is also known in the art. By way of illustration, U.S. Pat. No. 4,238,404 discloses a process for preparing isocyanates from the Hofmann rearrangement under anhydrous conditions using tertiary amine bases, but the disclosures of this patent are limited to isocyanates wherein the isocyanate groups are bonded to secondary or tertiary carbons. No disclosure of the production of any isocyanates with the isocyanate groups bonded to primary carbons is made in the '404 patent.
New non-phosgene methods for producing isocyanates in high yield from amines, especially methods that are essentially water-free and capable of producing primary isocyanates, would be highly desired by the urethanes manufacturing community.